Ingot mould for quenching metals and ingots thus obtained

ABSTRACT

An ingot mold for metals which is intended for the production of an ingot through the cooling and solidification of a liquid metal mass. The mold includes a wall which defines a base and an inner surface having a part S, known as the cooling surface, which can discharge all or part of the heat energy released by the metal mass during the cooling and solidification thereof. The cooling surface S includes at least one flat surface element Si which forms all or part of the base ( 4 ) of the ingot mold. There is also at least one point C on a plane Pi which is tangential to each surface element Si, such that all of the segments of a straight line D connecting every point R of the cooling surface S to point C are only located inside the mold. Further, the total area of surface elements Si is at least equal to 10% of the surface S. The ingot mold can be used to accelerate the production of ingots.

FIELD OF THE INVENTION

This invention relates to casting of non-ferrous metals, andparticularly aluminium and its alloys. In particular, it relates tometal ingots and particularly stackable ingots, and the ingot mouldsused to obtain them.

STATE OF THE ART

Metal ingots are produced by pouring liquid metal into an ingot mouldwith a specific shape. The liquid metal cools, solidifies and producesan ingot with the same shape as the inside volume of the ingot mould.

Most ingots have a shape that facilitates storage by stacking andhandling of the stacks thus obtained. The stacks may be stabilised byone or several straps. In general, ingots are also provided with meansof limiting the volume of stacks and for self-stabilising them. Thesemeans are typically interlocking means such as projecting elements(studs, bosses, pads, etc.) and recessed elements (notches, grooves,etc.) that cooperate so that each ingot may be held in place by adjacentingots. Several shapes of ingot and ingot moulds have been proposed suchas those described in Pechiney's French patent FR 1 310 651(corresponding to U.S. Pat. No. 3,161,477), American Magnesium Co.'sU.S. Pat. No. 3,570,664, Ormet Corp.'s U.S. Pat. No. 3,498,451 and U.S.Pat. No. 3,671,204, Intalco Aluminium Corp.'s French patent FR 2 068 802(corresponding to U.K. patent application GB 1 315 134), the SovietUnion patent SU 1 065 076 taken out by the U.S.S.R. Institute ofScientific Research and Technical Studies for the aluminium, magnesiumand electrodes industry, and Sollac's French application FR 2 678 185.

Problem that Arises

The rate of the ingot manufacturing process including cooling andsolidification of ingots is a determining factor in the productivity ofa foundry. Thus, dissipation of heat from the metal contained in ingotmoulds in industrial metal ingot production systems is usuallyaccelerated using a cooling fluid, typically water, that is brought intothermal contact with the outside surface of the ingot moulds. Howeverdue to the permanent increase in the production capacity of metalproduction plants, and particularly in electrolytic aluminium productionplants, ingot manufacturing may become a step limiting the production ofa plant. Consequently, a permanent search is made for solutions toaccelerate manufacturing of ingots, while maintaining the quality ofingots obtained and the possibility of stacking them in a stable manner.

DESCRIPTION OF THE INVENTION

An object of the invention is a metal ingot mould designed forfabrication of ingots by cooling and solidification of a mass of liquidmetal with an initial volume Vo, comprising an inside cooling surface Sthat will dissipate all or some of the heat energy released by the massof liquid metal during cooling and solidification, and characterised inthat the shape of the cooling surface S is such that when the volume Voof metal contracts due to cooling and solidification, the metal remainsin contact with at least 10% of the surface area S.

Preferably, the metal remains in contact with at least 15% of thesurface area S and more preferably at least 20% of the surface area S.

In his search for solutions to the problem that arises with theinvention, the applicant has observed that unexpectedly, the effectivecooling time of ingots, from the pouring of the liquid metal into theingot mould until the extraction of the solidified ingot, is actuallysignificantly longer than predicted by estimates made from thermalcalculations, and that the importance of this phenomenon depends verymuch on the shape of the ingot mould. The applicant then had the ideathat the increased cooling time could largely be explained by a problemof thermal contact between the metal and the ingot mould and noted thatunexpectedly, contraction of the metal during its solidification createsa slight separation between the ingot and the inside surface of theingot mould at many locations. Although small, this separation createsan air film that significantly reduces heat exchanges between the ingotand the wall of the ingot mould. Heat exchanges then only take placeover very small areas at the interface between the ingot and the ingotmould.

In one preferred embodiment of the invention, the metal ingot mould ischaracterised in that the cooling surface comprises at least one planesurface element Si preferably forming all or part of the bottom of theingot mould, and in that there is at least one point C on a plane Titangent to the, or to each, surface element Si such that all straightline segments D connecting any point R on the cooling surface S to thepoint C pass only inside the ingot mould, and in that the total surfacearea of the surface element or elements Si is equal to at least 10% ofthe cooling surface area S.

Preferably, the total surface area of the surface element or elements Siis equal to at least 15% of the cooling surface area S, and even morepreferably at least 20% of the cooling surface area S.

Another object of the invention is a metal ingot that could be obtainedwith an ingot mould according to the invention, comprising a mouldedsurface Sm and a rough surface Sb, and characterised in that the mouldedsurface Sm comprises at least one plane surface element Si, in thatthere is at least one point C on a plane Pi tangent to the, or to each,surface element Si such that all straight line segments D connecting anypoint R on the moulded surface Sm to the point C pass only inside theingot, and in that the total surface area of the surface element orelements Si is equal to at least 10% of the moulded surface area Sm.

The moulded surface area Sm corresponds to the part of the total surfaceof the ingot that was formed by the ingot mould, namely the initialsurface So. The remainder of the surface of the ingot or the roughsurface Sb, typically corresponds to the upper part of the initial massof liquid metal.

Preferably, the total surface area of the surface element or elements Siis equal to at least 15% of the moulded surface area Sm, and morepreferably equal to at least 20% of the moulded surface area Sm.

Another object of the invention is the use of an ingot mould accordingto the invention for manufacturing of metal ingots.

Another object of the invention is a method for manufacturing metalingots using an ingot mould according to the invention.

The invention is particularly suitable for manufacturing of non-ferrousmetal ingots and particularly ingots made of aluminium, aluminium alloy,magnesium, magnesium alloy, zinc or zinc alloy.

The invention will be better understood after reading the attachedFigures and the detailed description given below that describe apreferred embodiment.

FIGS. 1 and 2 show longitudinal sectional views showing two typicalingot moulds according to prior art and the effect of contraction of themetal as it cools and solidifies.

FIG. 3 shows an ingot mould according to the invention.

FIG. 4 shows an ingot mould according to the invention seen in alongitudinal sectional view, and the effect of contraction of the metalas it cools and solidifies.

FIG. 5 shows profiles of ingot moulds according to variants of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

As can be seen in the attached Figures, an ingot mould (1) typicallycomprises a wall (2) usually made of metal and/or a refractory material,and an opening (3) through which liquid metal can be poured into theingot mould. The wall (2) defines a bottom (4), sidewalls (2′) and endwalls (2″). The wall (2) has an inner surface (5) and shape elements (6,7, 8) that will apply a determined shape to the ingot. In particular,these shape elements produce ingot interlocking or handling elements.

The liquid metal (10) initially fills a volume Vo and comes into contactwith the wall (2) over a part So of the internal cooling surface S. Theratio between the area Ao of the surface So and the volume Vo of theliquid metal is then high, typically of the order of 0.5 cm⁻¹. Duringcooling and solidification, the metal contracts (occupying a volume Vo′smaller than Vo) and separates from the wall in several locations, thusforming air films (9). As shown in FIGS. 1 and 2, in ingot mouldsaccording to prior art, the area Ar of the residual contact surface Sris significantly smaller than the initial area Ao. The applicantestimates that the area of the residual surface obtained with ingotmoulds according to prior art is significantly less than 10% of theinitial area (typically of the order of 5%). Consequently, a smallreduction in volume Vo will cause a considerable increase in the thermalresistance.

According to the invention, a large contact area can be maintaineddespite contraction of the metal, due to the use of an appropriate shapeof the inside surface of the ingot mould. The shape is preferably suchthat when the volume Vo of metal contracts due to cooling andsolidification, the metal remains in contact with at least 10% of thecooling surface area S.

In one preferred embodiment of the invention, the metal ingot mould (1)that will be used for manufacturing an ingot (11) by cooling andsolidification of a mass of liquid metal (10), comprises a wall (2) andan opening (3), the said wall (2) defining a bottom (4) and an insidesurface (5) of which a part S, called the cooling surface, can dissipateall or some of the heat energy released by the metal mass (10) duringcooling and solidification, the said wall (2) comprising at least oneshape element (6, 7, 8) that will form at least one interlockingelement, one stacking element or one handling element on the ingot (11),and is characterised in that the cooling surface S comprises at leastone plane surface element Si forming all or part of the bottom (4) ofthe ingot mould (1), in that there is at least one point C on a plane Pitangent to the surface element, or to each surface element, Si such thatall straight line segments D connecting any point R on the coolingsurface S to point C pass only inside the ingot mould (1), and in thatthe total surface area of the surface element or elements Si is equal toat least 10% of the cooling surface area S.

In other words, the straight line segments D do not touch any otherpoint on the surface S, except surface elements Si.

Preferably, the total surface area of the surface element or elements Siis equal to at least 15% of the surface S, and more preferably at least20% of the surface S.

The impact of contraction of the metal caused by cooling andsolidification of the liquid metal (10) that is initially in contactwith a part So of the cooling surface S may be visualized approximatelyas a homothetic contraction of the surface So by a relatively smallquantity K from point C. In FIG. 4, it can be seen that in an ingotmould according to the invention, contraction does not generate anyintersection between the contracted surface So′ thus obtained and theinitial surface So so that the area of each surface Si of the bottom (4)can be kept practically unchanged (in the case shown in FIG. 4, thebottom comprises two surfaces Si that are identified by marks S₁ and S₂in FIG. 3). In fact, the homothetic contraction keeps the contractedsurface So′ in contact with the surface elements Si by sliding on theirplane Pi. When there is more than one surface element Si, the point C isat the intersection of the corresponding planes P1, P2, . . . , as shownin FIG. 3.

The effect of gravitation is taken into account by the fact that thesurface element(s) Si is (are) located at the bottom of the ingot mould.In practice, the point C is preferably such that the centre of mass ofthe contracted volume Vo′ corresponding to the contracted surface So′ isat the lowest possible point with respect to the normal direction of useof the ingot mould, in other words it is impossible to move thecontracted surface So′ vertically downwards without creating anintersection between So′ and the inside surface (5) of the ingot mould.In other words, the proportional contraction leaves the contractedsurface So′ at the lowest gravitational level with respect to thedirection of use of the ingot mould. The ingot moulds according to theinvention can thus maintain a considerably greater residual contactsurface than ingot moulds according to prior art.

The exact value of the quantity K called the “proportionaltransformation ratio” is not critical for operation of the invention,provided that it represents thermal contraction values obtained withmetals. It is sufficient to use a proportional transformation ratio Kless than about 1% to determine appropriate cooling surface shapes.Contractions in the metal volume from Vo to Vo′ shown in the attachedFigures have been deliberately exaggerated to better illustrate theprinciple of the invention.

Surface elements Si are advantageously at an angle α_(i) with respect tothe normal initial level N of the liquid metal (10). The said level N istypically parallel to the outside edge (16) of the opening (3) of theingot mould (1). The angle α_(i) is preferably less than 30° and morepreferably less than 20° in order to optimise the volume of the ingotwhile releasing a space under it through which a strap can be passedwhen stacking the ingots obtained.

The cooling surface S normally comprises more than five distinct surfaceelements Si, namely at least two sidewalls (2′), two end walls (2″) anda bottom (4), so as to form the shape elements (6, 7, 8, 14, 15). Forexample, the ingot mould shown in FIG. 3 comprises at least ten distinctsurface elements (including the sidewalls (2′)).

The ingot mould according to the invention typically comprises an evennumber of surface elements Si. The number of surface elements Si ispreferably equal to 2 (as shown in FIGS. 3 and 4) in order to simplifyits production and to more easily obtain a very large residual contactsurface. The surface elements Si are preferably contiguous (as shown inFIG. 3) so as to maximise the residual contact surface.

FIG. 3 shows one embodiment of the invention which is particularlyadvantageous in which there are two surface elements Si denoted S₁ andS₂, that are not in the same plane and that intersect at point C. FIG. 5shows variants of the invention in which the bottom (4) comprisesadditional shape elements (14, 15).

The surface elements Si may have different areas Ai and may be inclinedat a different angle α_(i). In order to simplify the production and useof the ingot mould according to the invention, it advantageously has aprincipal axis A and a plane of symmetry B perpendicular to itsprincipal axis A, and the point C is located in the plane of symmetry B.In this embodiment, the angle α_(i) is the same for surface elements Siarranged symmetrically. In this case, the outside edge (16) of theopening (3) of the ingot mould (1) is preferably approximately straightand perpendicular to plane B and the initial normal level N of theliquid metal (10) is approximately parallel to the said outside edge(16).

Preferably, none of the angles between the inside surface elements ofthe ingot mould is less than 90°, to avoid forming areas that wouldblock the ingot in the ingot mould and would make extraction difficult.

Locking elements typically comprise projecting elements (studs, bosses,pads, etc.) and recessed elements (notches, grooves, etc.) thatcooperate with each other so that each ingot can be retained by adjacentingots. Stacking elements typically comprise projecting or recessedelements (such as depressions) so that ingots can be stacked in anoptimum manner, and/or so that stack stabilisation such as straps can beplaced. Handling elements typically include projecting and/or recessedelements that form gripping means such as “lugs” or handles.

Another object of the invention is a metal ingot (11) comprising amoulded surface Sm and a rough surface Sb, comprising at least oneelement chosen from among interlocking elements, stacking elements andhandling elements, and characterised in that the moulded surface Smcomprises at least one plane surface element Si, in that there is atleast one point C on a plane Pi tangent to the surface element or toeach surface element Si such that all straight line segments Dconnecting any point R on the moulded surface Sm to point C pass onlyinside the ingot (11), and in that the total surface area of the surfaceelement or elements Si is equal to at least 10% of the moulded surfacearea Sm.

Thus, like the case of the ingot mould according to the invention, aproportional contraction of the surface Sm by a quantity K determinedwith respect to point C, does not create any intersection between thecontracted surface Sm′ thus obtained and the moulded surface Sm.

Preferably, the total surface area of the surface element or elements Siis equal to at least 15% of the moulded surface Sm, and more preferablyequal to at least 20% of the moulded surface Sm.

Each surface element Si is advantageously inclined by an angle α_(i)from the rough surface Sb of the ingot, which can optimise the volume ofthe ingot while releasing a space under the ingot around which a strapcan be placed when stacking ingots. The angle α_(i) is preferably lessthan 30° and more preferably less than 20°. The applicant has noted thatthe free space thus obtained is particularly advantageous because itmeans that a strap made of a flexible material such as polyester can beused, that holds the stack in position very well when the ingots arestacked without risk of it wearing during handling of the stack. If thisfree space is not present, the strap can rub on the floor and wear byabrasion. It is usually sufficient for the depth H of the free spaceunder the ingot obtained to be between 6 and 12 mm for an approximately70 cm long ingot.

The ingot according to the invention typically includes an even numberof surface elements Si preferably two surface elements Si to simplifyits manufacturing. In this case, the two surface elements Si aretypically contiguous.

In one advantageous embodiment of the invention, the ingot has aprincipal axis A and a plane of symmetry B perpendicular to itsprincipal axis A, and the point C is in the plane of symmetry B. In thisembodiment, the angle α_(i) is the same for surface elements Si arrangedsymmetrically. The number of surface elements Si is preferably equal to2 (as shown in FIGS. 3 to 5). The surface elements Si are preferablycontiguous (as shown in FIGS. 3 and 4).

In order to facilitate handling of ingots according to the invention,they preferably include handling elements (13), and typically two endelements called “lugs” as shown in FIG. 4.

The ingot according to the invention is typically a stackable ingot thatmay be obtained using the ingot mould according to the invention.

Another object of the invention is a method of manufacturing metalingots in which a volume Vo of the liquid metal is poured into an ingotmould according to the invention, the ingot mould is subjected to a flowof cooling fluid (typically water) and the ingot is extracted aftercooling and solidification of the metal.

The metal is typically aluminium, an aluminium alloy, magnesium, amagnesium alloy, zinc or a zinc alloy.

The invention can be used to obtain ingots free of bubbles and crackscaused by shrinkage of metal as it cools.

It also prevents ingots from getting blocked in the ingot mould bythermal contraction. Stripping of the ingots is made easier which alsocontributes to accelerating ingot manufacturing operations.

Tests

Comparative tests were carried out with metal ingot moulds similar tothose shown in FIG. 2 (prior art) and FIG. 3 (invention). The metal wasaluminium. The amount of cast metal was typically 23 and 28 kg.

The solidification times were more than 350 s for ingot moulds accordingto prior art and of the order of 335 s for ingot moulds according to theinvention. The solidification times obtained with ingot moulds accordingto prior art were highly dispersed (standard deviation more than 30 s)whereas they were not very dispersed with ingot moulds according to theinvention (standard deviation less than 3 sec). Ingots obtained withingot moulds according to the invention were generally free of shrinkageand cracks.

The total inside surface of ingot moulds (including sidewalls (2′))according to prior art and according to the invention was about 2300cm². The applicant estimates that the value of the residual contactsurface area was about 5% of the total surface area for ingot mouldsaccording to prior art and about 20% of the total surface area for ingotmoulds according to the invention.

LIST OF DIGITAL MARKS

-   1 Metal ingot mould-   2 Wall-   2′ Sidewalls-   2″ End walls-   3 Opening-   4 Bottom-   5 Inside surface-   6, 7, 8 Shape elements-   9 Air films-   10 Liquid metal-   11 Ingot-   12 Liquid metal free surface-   13 Handling elements-   14, 15 Shape elements-   16 Outside edge of ingot mould opening

1. Metal ingot mould (1) designed for manufacture of an ingot (11) bycooling and solidification of a mass of liquid metal (10), comprising awall (2) and an opening (3), the said wall (2) defining a bottom (4) andan inside surface (5) of which a part S, called the cooling surface, candissipate all or some of the heat energy released by the metal mass (10)during cooling and solidification, the said wall (2) comprising at leastone shape element (6, 7, 8) that will form at least one interlockingelement, one stacking element or one handling element on the ingot (11),and characterised in that the cooling surface S comprises at least oneplane surface element Si forming all or part of the bottom (4) of theingot mould (1), in that there is at least one point C on a plane Pitangent to the, or to each, surface element Si such that all straightline segments D connecting any point R on the cooling surface S to pointC pass only inside the ingot mould (1), and in that the total surfacearea of the surface element or elements Si is equal to at least 10% ofthe cooling surface area S.
 2. Ingot mould according to claim 1, acharacterised in that the total surface area of the surface element orelements Si is equal to at least 15% of the cooling surface S.
 3. Ingotmould according to claim 1, characterised in that the total surface areaof the surface element or elements Si is equal to at least 20% of thecooling surface S.
 4. Ingot mould according to claim 1, characterised inthat each surface element Si is inclined by an angle α_(i) from theinitial normal level N of the liquid metal (10).
 5. Ingot mouldaccording to claim 4, characterised in that the angle α_(i) is less than30° and preferably less than 20°.
 6. Ingot mould according to claim 1,characterised in that it includes an even number of surface elements Si.7. Ingot mould according to claim 6, characterised in that it includestwo surface elements Si.
 8. Ingot mould according to claim 7,characterised in that the two surface elements Si are contiguous. 9.Ingot mould according to claim 1, characterised in that it has aprincipal axis A and a plane of symmetry B perpendicular to itsprincipal axis A, and in that the point C is located in the plane ofsymmetry B.
 10. Metal ingot (11) comprising a moulded surface Sm and arough surface Sb, comprising at least one element chosen from amonginterlocking elements, stacking elements and handling elements, andcharacterised in that the moulded surface Sm comprises at least oneplane surface element Si, in that there is at least one point C on aplane Pi tangent to the surface element or to each surface element Sisuch that all straight line segments D connecting any point R on themoulded surface Sm to point C pass only inside the ingot (11), and inthat the total surface area of the surface element or elements Si isequal to at least 10% of the moulded surface area Sm.
 11. Ingotaccording to claim 10, characterised in that the total surface area ofthe surface element or elements Si is equal to at least 15% of themoulded surface Sm.
 12. Ingot according to claim 10, characterised inthat the total surface area of the surface element or elements Si isequal to at least 20% of the moulded surface Sm.
 13. Ingot according toclaim 10, characterised in that each surface element Si is inclined byan angle α_(i) from the rough surface Sb of the ingot.
 14. Ingotaccording to claim 13, characterised in that the angle α_(i) is lessthan 30 and preferably less than 20°.
 15. Ingot according to claim 10,characterised in that it includes an even number of surface elements Si.16. Ingot according to claim 15, characterised in that it includes twosurface elements Si.
 17. Ingot according to claim 16, characterised inthat the two surface elements Si are contiguous.
 18. Ingot according toclaim 10, characterised in that it has a principal axis A and a plane ofsymmetry B perpendicular to its principal axis A, and in that the pointC is located in the plane of symmetry B. 19-21. (canceled)
 22. Methodfor manufacturing metal ingots in which a volume Vo of liquid metal ispoured in an ingot mould according to claim 1, the ingot mould issubjected to a flow of cooling fluid and the ingot is extracted aftercooling and solidification of the metal.
 23. Manufacturing methodaccording to claim 22, characterised in that the metal is a non-ferrousmetal.
 24. Manufacturing method according to claim 23, characterised inthat the non-ferrous metal is chosen from among aluminium, aluminiumalloys, magnesium, magnesium alloys, zinc or zinc alloys.